This invention relates to a process for producing a blending vinyl chloride resin which is suitable for preparing a paste dispersion having superior viscosity properties and which can give molded articles having superior quality.
The important properties of paste vinyl chloride resins (to be referred to simply as a paste PVC) are such that paste dispersions (e.g., plastisols or organosols) formed by blending them with plasticizers, diluents, stabilizers, fillers, etc. have superior rheological properties and can be easily processed at various shear rates under which they are placed in processing (for example, from low shear rates in slush, rotational, dip, casting and sponge moldings to high shear rates in spreading and spray coating operations); they have sufficient flowability even when the amounts of plasticizers and/or diluents are small; and that their viscosities vary little with time. In short, paste PVC should give paste dispersions having superior viscosity properties (i.e., these paste dispersions should have as low an initial viscosity as possible with little change in viscosity over a long period of time). In order that paste dispersions may have superior viscosity properties, paste PVC should have an average particle diameter of 0.1 to 5 microns, preferably 0.2 to 3 microns, a suitable particle diameter distribution, and moderate affinity at its surface with plasticizers. The paste PVC should also be excellent in heat stability during processing and in the color, clarity, strength, etc. of final molded products. For this purpose, the paste PVC should have a low content of impurities.
General practices of producing paste PVC having these properties include, for example, a method which involves adjusting the particle diameter and particle diameter distribution of the paste PVC by the selection of an emulsifier, the improvement of the manner of adding the emulsifier, the use of a seed polymerization method and the selection of conditions for a homogenizer, a dryer or a pulverizer, and a method which is directed to the modification of the surface of the resin particles by adding an emulsifier (mainly a nonionic emulsifier) after polymerization. Paste PVC resins produced by these methods, however, have not proved to be entirely satisfactory in regard to the viscosity properties of paste dispersions prepared therefrom.
In order to improve the viscosity properties of paste dispersions, it is the usual practice to blend such paste PVC with coarse particles of vinyl chloride resins. These vinyl chloride resins in the form of coarse particles are generally called "blending PVC" or "extender PVC". First of all, the blending PVC should preferably be in the form of spherical single particles having a smooth surface in order to increase the effect of reducing the viscosity of paste dispersions. The blending PVC can bring about a greater effect of viscosity reduction as its particle diameter is larger. On the other hand, larger particle diameters cause disadvantages such as the degradation of properties and the reduction of clarity owing to insufficient melting during processing, and the sedimentation of polymer particles in paste dispersions. To avoid these disadvantages, the blending PVC should preferably have an average particle diameter of 10 to 80 microns, particularly 20 to 60 microns, with its particle diameter distribution being not too broad. Furthermore, the surfaces of the particles of the blending PVC should preferably melt easily during processing and have affinity for paste PVC in order to obtain molded articles having excellent strength and clarity.
It is an object of this invention to provide a process for producing a blending PVC having the aforesaid excellent properties.